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Mining the urban waste

Until 2003, the prices of an enormous range of metals had remained amazingly constant for decades. It may not have felt like that at the time, with prices varying by up to 50% year-on-year. But with the benefit of hindsight it is clear that, over the longer term, average prices of a huge range of materials remained relatively stable.

But from 2003, a step change took place, with raw materials prices increasing sharply for five years. In 2008 they reached a peak, before the credit bubble burst and prices tumbled.

However, even at the deepest point of the credit crunch, commodity prices were still around double that of the previous long-term average, and it did not take long for them to rebound.

The main trigger for the rapid price rises and longer resilience has been the growth of developing economies. This has resulted in a significant increase in demand for all kinds of industrial raw materials. There have also been sharp reminders recently of the potential fragility of supply of some key metals.

Adding to this volatility, in August 2012 a simmering labour dispute at a South African platinum mine spilled over into violence, leading to several deaths. A series of damaging wildcat strikes at a number of platinum and gold mines followed, along with mass sackings and the threat of mine closures.

Strategic resources

Only a few weeks later, a rare earth metal refinery in Malaysia, that would have helped to break the Chinese stranglehold on the supply of these technologically important metals, had its operating licence delayed because of environmental concerns.

Thus the subject of materials scarcity and strategic resources has risen to public prominence.

The industrialised West has been forced to realise that many of the key metals and materials on which its modern way of life depends are not infinite, and may not be available indefinitely.

Column inches on the subject have certainly not been scarce in the past few months, as regulators, consultants and industry organisations have rushed to come to terms with the situation. These are interesting times for waste handlers, producers and others in the industry.

Economic, environmental, political and social pressures have fallen into step, and there is real hope that progress can be made in reducing our use of raw materials in the future. Cutting our dependence on imported raw materials requires us to reduce the amount we use in the first place, through changes in product design and manufacture.

The next step is to improve the recycling and recovery of materials from waste. Where there’s muck, there’s not just brass but a whole range of strategic and scarce metals as well.

The total value of platinum group metals (PGM) in discarded catalytic converters in the UK is around £250m a year; the total value of copper and precious metals (PM) in electronic goods worldwide is more than £20bn annually.

Such opportunities for value recovery from waste have led to a growth in interest in recycling and recovery technologies. For example, plasma technology has been adopted all over the world for the recovery of PGM from catalysts. And it seems clear that the next related growth area will be the extraction of copper and PM from electronics waste.

Recovery optimisation

Tetronics is working on integrating its plasma technology with advanced shredding and sorting techniques (see box) to optimise the recovery of not only PM and copper, but also rare earth elements and other valuable materials from electronics waste. This will ensure the highest possible extraction of valuable metals from this important ‘urban mine’.

This trend for urban mining will be good for the environment and lead to important social and economic outcomes. In a world where raw materials are rising in value, production of technologically important metals from any source will bring real economic benefits to governments and communities.

Dr Tim Johnson, technical director at Tetronics International

How plasma technology works

Tetronics International’s DC plasma arc technology has been developed for the recovery of valuable raw materials from wastes. A combination of flexible operation and tight control of smelting conditions make it suitable for separating the valuable and less valuable fractions of the waste.

The low gas flows typical of plasma processes make it possible to process materials with minimal losses of value-containing dust to the exhaust gases.

A small footprint also enables plasma systems to be retrofitted to existing facilities and to be installed close the source of the waste, which reduces the environmental impact of the technology even further.

In a typical plasma waste treatment system, the waste material is blended with fluxing additions to lower the melting point of the slag, and/or with reducing agents to recover reducible metals from the waste.

The plasma furnace consists of a refractory lined vessel, similar in construction to an electric arc furnace, with either a graphite electrode or a water-cooled plasma torch as the plasma device. The electrical power for the arc is provided by the plasma power supply, which converts the incoming AC voltage supply into the controlled DC supply required for the plasma.

As with normal polarity welding, the molten bath is usually the anode of the circuit and the graphite electrode or plasma torch is usually the cathode.

The plasma arc attaches to the surface of a bath of slag, which floats on a layer of liquid metal in the bottom of the plasma furnace, both of which are typically at temperatures of 1,400C to 1,600C. The waste material feed-falls on to this molten bath, causing the moisture and volatile species such as organics, metals and metal halides to vaporise.

These are drawn off into the exhaust gases at 1,100C to 1,200C before passing through a thermal oxidiser to the gas cleaning system. The exhaust gases are then cleaned of acid gases before compliant discharge to atmosphere. In cases where chlorine levels are sufficiently high, it is also possible to collect the chlorine as hydrochloric acid in the gas cleaning system for sale as a pickling acid.

It is also possible to inject oxidants into the furnace to encourage gasification of organic components. The remaining non-volatile components, mainly metal oxides, either melt into the slag or are reduced to free metals and collect in the metal layer below.

In addition to the recovery of valuable metals, the Plasmarok product of the process can be used as an aggregate in a wide range of traditional building applications, due to its inert low leaching character and mechanical properties equivalent to or better than natural materials such as granite.

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